In spite of a small and indirect bandgap, silicon features properties that are potentially very attractive for optical applications. The unsurpassed level of impurity control leads to suppression of nonradiative recombination paths and provides for a very long minority carrier lifetime. Up to the present, observation of an optical memory effect has not been reported for crystalline silicon (c-Si). In contrast, it has been observed in GaAs and GaN. In this study, we have used c-Si doped with erbium (Er) ions, since the trivalent Er3+ ion emits at a wavelength of 1.54 μm that is suitable for telecommunication applications. In a two-color experiment, utilizing a Nd:YAG pulse excitation as a writing beam and a free-electron laser (FEL) as a reading beam, we revealed that Si exhibits optical memory and afterglow effects at low temperatures T<50 K. Both effects are, in fact, manifestations of the properties of Si itself, conveniently revealed by the optical dopant. Detailed investigations of the FEL-induced emission at 1.54 μm show that this excitation mechanism is governed by the release of a single type of charge carrier. This implies that hole and electron capture events necessary for the Er excitation could, in principle, be separated in time. Therefore it appears possible to combine optical and electrical write/read procedures for the reported memory effect.
In spite of a small and indirect bandgap, silicon features properties that are potentially very attractive for optical applications. The unsurpassed level of impurity control leads to suppression of nonradiative recombination paths and provides for a very long minority carrier lifetime. Up to the present, observation of an optical memory effect has not been reported for crystalline silicon (c-Si). In contrast, it has been observed in GaAs and GaN. In this study, we have used c-Si doped with erbium (Er) ions, since the trivalent Er3+ ion emits at a wavelength of 1.54 μm that is suitable for telecommunication applications. In a two-color experiment, utilizing a Nd:YAG pulse excitation as a writing beam and a free-electron laser (FEL) as a reading beam, we revealed that Si exhibits optical memory and afterglow effects at low temperatures T<50 K. Both effects are, in fact, manifestations of the properties of Si itself, conveniently revealed by the optical dopant. Detailed investigations of the FEL-induced emission at 1.54 μm show that this excitation mechanism is governed by the release of a single type of charge carrier. This implies that hole and electron capture events necessary for the Er excitation could, in principle, be separated in time. Therefore it appears possible to combine optical and electrical write/read procedures for the reported memory effect.
Erbium is a rare-earth metal that, when incorporated in a solid, can emit light at a wavelength of 1.5 μm. It plays a key role in current day telecommunication technology as the principle ingredient of optical fiber amplifiers. In this thesis the control of the Er spontaneous emission in three different types of microphotonic materials is described. Part I of this thesis focuses on the effect of a metallo-dielectric interface on the spontaneous emission of optical emitters in silica glass. It is shown that Er ions near a Ag interface can couple to surface plasmons (SPs) via a near-field interaction. By coupling SPs out into the far field, large changes in the Er photoluminescence emission distribution, spectra, and polarization can be observed. The excitation of SPs also results in an increase of the Er photoluminescence decay rate. The observed decay rates are in good agreement with calculations based on a classical dipole oscillator model. From the change in photoluminescence decay rate of Si nanocrystals near a Ag interface it is shown that Si nanocrystals can efficiently excite SPs and have an internal quantum efficiency of 77 %. Part II focuses on the effect of a microcavity on the spontaneous emission of Er and describes how ion implantation can be used to dope dielectric microresonators with optically active Er ions. The fabrication and characterization of an Er ion-implanted silica microsphere resonator is described that shows lasing at 1.5 μm when pumped above its lasing threshold. Ion implantation is also used to dope toroidal microcavities on a Si chip with Er. The microtoroids are doped by either pre-implantation into the SiO2 base material, or by post-implantation in a fully fabricated microtoroid. The optical activation of Er ions in the microtoroid is investigated and Er lasing at 1.5 μm is observed for both types of microcavities with the lowest threshold (4.5 μW) for the pre-implanted microtoroids. Part III describes the fabrication of an Er-doped Si-inverse opal photonic crystal. These photonic crystals can potentially have a photonic bandgap that can fully inhibit the spontaneous emission of on optical emitter. Fabrication criteria are derived for such a photonic crystal, based on the lattice parameter, filling fraction, and Si refractive index. In the opal photonic crystal composed of both Si and SiO2 we show that Er ions can be selectively excited in both the Si and SiO2 part of the photonic crystal by changing the excitation wavelength and/or the measurement temperature.
Energy transfer in erbium doped optical waveguides based on silicon This thesis describes the energy transfer processes\noccurring in materials that can be used for the fabrication of silicon compatible optical integrated circuits, operating at 1.54 mm.The thesis consists of three parts: Part I discusses energy transfer in optical waveguide amplifiers on silicon. The waveguide amplifiers use an intra-4f transition of the rare earth ion erbium occurring at 1.54 mm. To achieve significant amplification, high\n Er concentrations (> 10 20 cm -3 ) are required. At these concentrations Er ions can interact, resulting in co-operative\n upconversion, in which one excited Er ion is excited into a higher energy state, while a neighboring Er ion returns to the ground\n state. In Part I this and other gain limiting effects are investigated. In particular, it is shown that the atomic-scale Er distribution\n is of crucial importance for the performance of Er doped waveguide amplifiers. Part II discusses the energy transfer processes\n occurring in Er doped silicon. It is shown that Er in silicon can be excited by electron-hole pair recombination in an\n impurity-Auger effect. This effect may be used to fabricate an electrically pumped Er doped waveguide amplifier. The reverse\n process can also occur: an excited Er ion can de-excite and generate an electron-hole pair in the silicon, due to a non-radiative\n backtransfer process. This process has an efficiency of 70% at room temperature. This observation enables the fabrication of an\n Er doped photo-detector in a Si waveguide operating at 1.54 mm. The performance of such a detector is discussed. Part III\n discusses the interaction between Er and silicon nanocrystals embedded in SiO2. Si nanocrystals efficiently absorb visible light,\n resulting in the generation of quantum confined electron-hole pairs. The electron-hole pairs efficiently transfer energy to Er.\n Due to the energy transfer, the effective Er absorption cross section is five orders of magnitude larger than for direct optical\n excitation. Silicon nanocrystals may thus be used to increase the pumping efficiency of an Er doped waveguide amplifier. It is\n shown that a single nanocrystal can excite ~1 Er ion at a time and that an optical gain of 0.6 dB/cm may be achieved in a typical\n nanocrystal sensitized waveguide amplifier. Due to the broad absorption band of Si nanocrystals, a broad band light source can\n be used as pump source in such a nanocrystal sensitized Er doped waveguide amplifier.
Rare-earth ions in solid-state hosts exhibit low homogeneous broadening and long spin coherence at cryogenic temperatures thus making them a promising candidate for optical quantum memories [1], optical-microwave transducers [2] and single dopant-based devices such as single photon emitters operating at the telecommunication wavelengths [3]. Here, we present an overview of the spin and optical properties of Er ensembles in Si accessed via resonant photoluminescence excitation (PLE) spectroscopy [4]. Samples were positioned directly on top of tailor-fabricated superconducting single photon detectors, placed in a dilution refrigerator unit and resonantly excited using fiber optics. Our method provided high collection efficiency and allowed spectral measurements of low Er density samples. The observed Er PLE spectra were strongly affected by the presence of co-dopants such as O, B or P reflecting their influence on the formation of optically active Er sites and corresponding Er optical transition energies. Higher O densities resulted in especially rich PLE spectra suggesting that a variety of Er-O complexes formed due to O doping. Lowering the O concentration, we achieved PLE spectra dominated by a single Er site in P doped samples. Interestingly, the Er spin relaxation and optical coherence properties were barely affected by the employed co-dopants and mainly depended on the Er concentration. By lowering the Er concentration from relatively high (1018cm-3) to low densities (1016cm-3), we were able to increase Er spin lifetimes from ~0.1 s to ~30 s. Long spin relaxation times allowed us to identify PLE spectra of new Er sites in Si. In both concentration regimes, we observed sub-MHz transient spectral holes suggesting long optical coherence times. Furthermore, we observed sub-MHz spin transitions in both natural and isotopically purified Si and achieved spin coherence times above 1 ms in isotopically purified Si. Narrow optical linewidths and long transverse and longitudinal spin relaxation times show that Er in Si is an excellent candidate for future quantum information and communication applications.
Rare-earth ion dopants in solid-state hosts are ideal candidates for quantum communication technologies, such as quantum memories, due to the intrinsic spin–photon interface of the rare-earth ion combined with the integration methods available in the solid state. Erbium-doped cerium oxide (Er:CeO2) is a particularly promising host material platform for such a quantum memory, as it combines the telecom-wavelength (∼1.5μm) 4f–4f transition of erbium, a predicted long electron spin coherence time when embedded in CeO2, and a small lattice mismatch with silicon. In this work, we report on the epitaxial growth of Er:CeO2 thin films on silicon using molecular beam epitaxy, with controlled erbium concentration between 2 and 130 parts per million (ppm). We carry out a detailed microstructural study to verify the CeO2 host structure and characterize the spin and optical properties of the embedded Er3+ ions as a function of doping density. In as-grown Er:CeO2 in the 2–3 ppm regime, we identify an EPR linewidth of 245(1) MHz, an optical inhomogeneous linewidth of 9.5(2) GHz, an optical excited state lifetime of 3.5(1) ms, and a spectral diffusion-limited homogeneous linewidth as narrow as 4.8(3) MHz. We test the annealing of Er:CeO2 films up to 900 °C, which yields narrowing of the inhomogeneous linewidth by 20% and extension of the excited state lifetime by 40%.
Since the advent of vertical cavity surface-emitting lasers, a renewed interest has also emerged for the spontaneous emission from Fabry-Pérot microcavities. Changes in spontaneous emission characteristics in a 1D microcavity were predicted long ago,1 but have been demonstrated only recently in the optical domain for solid-state, e.g. semiconductor, structures where changes in spectral purity, lifetime, and emission intensity were observed.2 However, the interpretation of these effects is somewhat obscure, because these structures are highly absorbing and often involve band-to-band, rather than atomic, transitions. In the present study, we use optically transparent Si/SiO2 cavities, consisting of two coplanar Si/SiO2 distributed Bragg reflectors (DBR) around a (λ/2) SiO2 active region, which was doped with ~0.2 at.% Er3+ ions through MeV ion implantation. The Er3+ ions in SiO2 exhibit a sharp, atomic-like, intra-4f transition around λem=1.54 μm, which is effectively shielded by the outerlying filled 5s2 and 5p6 shells. Recently, we reported on the considerable enhancement of the spontaneous emission intensity out of such an Er-doped microcavity.3 In this paper, we present a more detailed study of the modified spontaneous emission, including fluorescence decay measurements to study the effect of the cavity on the spontaneous emission rate.
Epitaxial growth of erbium-doped cerium oxide (Er:CeO2) is achieved on Si (111) substrates by the cooperative integration of Si and oxide molecular beam epitaxy (MBE) technologies. Lattice matching between CeO2 and Si provides an attractive opportunity to build dilutely doped Er-based optical devices on a Si chip. The CeO2 host crystal is optically transparent for the telecom C-band wavelength and has quite a small magnetic moment, which serves as a disturbance-free environment for the two-level system formed in the doped Er. After the systematic optimization of the growth conditions for stoichiometric Er:CeO2; i.e., (Er + Ce)/O = 1/2, we varied the Er concentration in a range of 1 ~4%. The doped Er showed well-defined optical transitions at the wavelength of 1.533 μm irrespective of the Er concentration. With decreasing Er concentration, enhancement of the luminescence intensity, narrowing of the spectral width, and an increase in the radiative lifetime were observed. The results suggest that CeO2 on Si is promising as a platform for the doped-Er-based optical devices and their quantum optics applications.